Mini SSTC: “Arges”

Well, since me and a friend are still “working” on our spark gap tesla coil without any progress whatsoever for the past three years or so I decided I should make a small tesla coil myself just to finally see some sparks (altough I must’ve watched hundreds of videos on the subject, I have never seen a real operating tesla coil).

I chose to build a small SSTC (solid state tesla coil) for the following two reasons:

• I didn’t have a transformer
• Nothing excites me more than solid state power electronics (well maybe chicks… who knows)

I decided to name my coil “Arges”. I didn’t want to name it after Thor or Odin or something because everyone does that, so instead I named it after one of the three cyclopes who – in Greek mythology – provided Zeus with his thunderbolts. (This also has the advantage that if I’ll build more coils in the future I can just name them after the remaining cyclopes.)

Because I don’t have any experience on the subject yet I am basically copying Steve Ward’s SSTC5, making some changes because of different mains voltages and component availability.

Circuit

Because Steve Ward’s circuit is designed for 120 V mains I had to replace the MOSFETs with higher voltage rated ones. I opted for the IRFP460 because it’s a commonly available part and also a popular choice for this kind of application.

Also, since I can’t get the UCC MOSFET drivers I decided to use the MAX4420 (noninverting) and MAX4429 (inverting) instead. These have a lower current rating but from the information I gathered should be usable for this kind of work. They also lack an “enable” input for the interrupter to connect to, so I added an AND gate (in the form of a CD4081 IC) to act as an “enable” for the drivers.

Base

I cut a piece of 18 mm thick soft wood to 18*36 cm as a base for all the electronics to sit on. A nice little find was that the buffer capacitor I used (470 uF 400 V) would fit nice and tight into a 40 mm PVC end cap.
This proved to be a good solution for mounting the capacitor to the base and protecting pokey little fingers from the 230 V.

To mount the secondary I glued a disc to the base on which I slide the secondary. Friction holds it in place pretty well. As a finish I added a small “high voltage”-label and coated the base in a few layers of the same polyurethane based varnish that I used on the secondary.

Toroid

Altough for this design a toroid is optional, I decided to make one nonetheless because I think a tesla coil with a toroid just looks so much cooler.

To build the toroid, I first cut an MDF disc (4 mm thickness) with a diameter of 20 cm. Then I used a jigsaw to turn a piece of 32 mm diameter PVC pipe into several rings which I slotted so they could be slid onto the plate. I used hot glue to fix them in place.

Then I made some papier mâché and put it on the construction to make the final toroidal shape. This was then dried in the oven at 50 °C for a few hours after which I took it out, added a little bit more papier mâché in some places that weren’t very good and let it dry at room temperature overnight.

To finish the toroid I glued aluminium foil to it and added a breakout point (fancy word for a screw) in the center. It will be mounted on the secondary by means of a wooden disc I secured to the bottom of the toroid.

Secondary

The secondary is made from a piece of 110 mm diameter PVC pipe about 28 cm long. With the tools I have available at home I wasn’t able to cut the pipe straight so in the final assembly I have to figure something to hide this from the eye. Before winding, I sanded the pipe a bit and applied two coats of polyurethane varnish to make it nice and smooth.

The winding height is slightly less than 25 cm, which amounts to 950 – 1000 turns with the wire I used (30 AWG, 33 SWG, 0,25 mm core diameter).

After winding was finished, I applied eight coats of polyurethane varnish to the coil. This secures the windings in place and gives it a nice smooth finish. After all was finished I used almost 200 ml of varnish. Next time I think I’ll buy a more shiny kind of varnish as this stuff looks rather dull when applied in large quantities.

To help with the winding and varnishing, I modified my mom’s old sewing machine so to rotate the PVC form. All I did was removing a lot of parts trying to make it run more smoothly (actually, it runs less smoothly since I made the modifications but well…) and attaching a wooden disc to a part that rotated at a reasonable speed to attach the coil to. On the other side the coil is supported by a construction made of MDF and a skateboard wheel (since I can’t skate and my “motorized skateboard” project failed miserabely I thought this would at last give the wheel some useful purpose).

Primary

For a primary I use 12 turns of wire wound directly on the bottom of the secondary. 12 turns is pretty conservative, but I’ll wait with removing turns until every

Testing & tweaking

When all was put together, it was time to fire the coil up. At first, I didn’t have a variac and/or isolation transformer so I just connected it to the 230 V with a ballast (a big transformer) in series. This worked ok, but didn’t produce very big arcs. Connecting the coil directly to the mains however blew up the FETs, time after time. The most frustrating part was the fact that the FETs constantly went *bang* before I would’ve had the chance to notice something wasn’t right. For this reason I abandoned the project for several months until I would’ve acquired an isolation transformer and a variac. But when I finally got hold of the right equipment, I was very eager to power the coil up again.

During testing, I made a small adjustment to the feedback system. Instead of antenna feedback I now use secondary feedback using a small ferrite transformer. I found this gives slightly better performance and reduces “hickups”.

This is the coil performing at 350 W, the highest power I’ve tried so far:

More videos can be found here.

Assorted pictures